Generally it is known from molecularbiology that, for the production of specific compounds, microorganisms are transformed with a so-called "artificial" plasmid, in that the genes, which are coded for this specific compound, are introduced. A special problem of these plasmids is their instability, i.e., their property not to be transmitted in a controlled way to the daughter cells during the cell division of the microorganisms. The result is that more and more daughter cells occur during the fermentation process which contain no plasmid or fewer of the plasmid.
On a laboratory scale this plasmid loss can be countered by supplying the antibiotic to the culture medium whose corresponding gene which is resistant to antibiotics contains the plasmid. However, the addition of the appropriate antibiotic in fermentations on a large scale has proven to be disadvantageous. Thus, for example, some antibiotics, such as, tetracycline, show unfavorable effects on the ability of microorganisms which contain the plasmid to grow, divide and reproduce Bioscience Reports, 5, (1985), pp. 29-37; Gene, 39, (1985), pp. 173-180!. Another drawback of antibiotic stabilization is that the addition of an antibiotic, especially in fermentation on a large scale, is too expensive. Further, the addition of an antibiotic in the production of pharmaceutical agents as well as in the production of food and feed additives is undesirable or unlawful.
Another method of how to counter this plasmid loss is described by H. Sakoda and T. Imanaka in J. Ferment. and Bioeng., Vol. 69, (1990), pp. 75-78. This method comprises a stable "recombinant host" plasmid system, in which first the tryptophan operon in the chromosome of the host is deleted and thus the host cell is inactive for the tryptophan transport. Then the host cell is transformed with a recombinant plasmid which carries this tryptophan operon. The selection of the host cells then takes place with this recombinant plasmid by means of the tryptophan transport. The drawbacks of this method are that, despite selection with tryptophan, in the actual fermentation process plasmid-free cells can also grow because of diffusion and thus daughter cells which contain no plasmid increasingly occur.